Methods for making environmental barrier coatings and ceramic components having CMAS mitigation capability

ABSTRACT

Methods of making components having calcium magnesium aluminosilicate (CMAS) mitigation capability include providing a component, applying an environmental barrier coating to the component, where the environmental barrier coating includes a CMAS mitigation composition selected from the group consisting of zinc aluminate spinel, alkaline earth zirconates, alkaline earth hafnates, rare earth gallates, beryl, and combinations thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/340,098, now U.S. Pat. No. 8,343,589, filed Dec. 19, 2008, which isherein incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments described herein generally relate to methods for makingenvironmental barrier coatings and ceramic components having CMASmitigation capability.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslybeing sought in order to improve their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. Significant advances inhigh temperature capabilities have been achieved through the formulationof iron, nickel, and cobalt-based superalloys. While superalloys havefound wide use for components used throughout gas turbine engines, andespecially in the higher temperature sections, alternativelighter-weight substrate materials have been proposed.

Ceramic matrix composites (CMCs) are a class of materials that consistof a reinforcing material surrounded by a ceramic matrix phase. Suchmaterials, along with certain monolithic ceramics (i.e. ceramicmaterials without a reinforcing material), are currently being used forhigher temperature applications. Using these ceramic materials candecrease the weight, yet maintain the strength and durability, ofturbine components. Furthermore, since such ceramic materials can havehigher temperature capability than metals, significant cooling airsavings can be realized that increase the efficiency of a turbineengine. Therefore, such materials are currently being considered formany gas turbine components used in higher temperature sections of gasturbine engines, such as airfoils (e.g. turbines, and vanes),combustors, shrouds and other like components that would benefit fromthe lighter-weight and higher temperature capability these materials canoffer.

CMC and monolithic ceramic components can be coated with environmentalbarrier coatings (EBCs) to protect them from the harsh environment ofhigh temperature engine sections. EBCs can provide a dense, hermeticseal against the corrosive gases in the hot combustion environment. Indry, high temperature environments, silicon-based (nonoxide) CMCs andmonolithic ceramics undergo oxidation to form a protective silicon oxidescale. However, the silicon oxide reacts rapidly with high temperaturesteam, such as found in gas turbine engines, to form volatile siliconspecies. This oxidation/volatilization process can result in significantmaterial loss, or recession, over the lifetime of an engine component.This recession also occurs in CMC and monolithic ceramic componentscomprising aluminum oxide, as aluminum oxide reacts with hightemperature steam to form volatile aluminum species as well.

Currently, most EBCs used for CMC and monolithic ceramic componentsconsist of a three-layer coating system generally including a bond coatlayer, at least one transition layer applied to the bond coat layer, andan optional outer layer applied to the transition layer. Optionally, asilica layer may be present between the bond coat layer and the adjacenttransition layer. Together these layers can provide environmentalprotection for the CMC or monolithic ceramic component.

More specifically, the bond coat layer may comprise silicon and maygenerally have a thickness of from about 0.5 mils to about 6 mils. Forsilicon-based nonoxide CMCs and monolithic ceramics, the bond coat layerserves as an oxidation barrier to prevent oxidation of the substrate.The silica layer may be applied to the bond coat layer, or alternately,may be formed naturally or intentionally on the bond coat layer. Thetransition layer may typically comprise mullite, barium strontiumaluminosilicate (BSAS), a rare earth disilicate, and variouscombinations thereof, while the optional outer layer may comprise BSAS,a rare earth monosilicate, a rare earth disilicate, and combinationsthereof. There may be from 1 to 3 transition layers present, each layerhaving a thickness of from about 0.1 mils to about 6 mils, and theoptional outer layer may have a thickness of from about 0.1 mils toabout 40 mils.

Each of the transition and outer layers can have differing porosity. Ata porosity of about 10% or less, a hermetic seal to the hot gases in thecombustion environment can form. From about 10% to about 40% porosity,the layer can display mechanical integrity, but hot gases can penetratethrough the coating layer damaging the underlying EBC. While it isnecessary for at least one of the transition layer or outer layer to behermetic, it can be beneficial to have some layers of higher porosityrange to mitigate mechanical stress induced by any thermal expansionmismatch between the coating materials and the substrate.

Unfortunately, deposits of CMAS have been observed to form on componentslocated within higher temperature sections of gas turbine engines,particularly in combustor and turbine sections. These CMAS deposits havebeen shown to have a detrimental effect on the life of thermal barriercoatings, and it is known that BSAS and CMAS chemically interact at hightemperatures, i.e. above the melting point of CMAS (approximately 1150°C. to 1650° C.). It is also known that the reaction byproducts formed bythe interaction of BSAS and CMAS can be detrimental to EBCs, as well assusceptible to volatilization in the presence of steam at hightemperatures. Such volatilization can result in the loss of coatingmaterial and protection for the underlying component. Thus, it isexpected that the presence of CMAS will interact with the EBC, therebyjeopardizing the performance of the component along with component life.

Accordingly, there remains a need for methods for making environmentalbarrier coatings and ceramic components having CMAS mitigationcapability.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments herein generally relate to methods of making componentshaving CMAS mitigation capability comprising: providing a component;applying an environmental barrier coating to the component, theenvironmental barrier coating comprising a separate CMAS mitigationlayer including a CMAS mitigation composition selected from the groupconsisting of zinc aluminate spinel, alkaline earth zirconates, alkalineearth hafnates, rare earth gallates, beryl, and combinations thereof.

Embodiments herein also generally relate to methods of making componentshaving CMAS mitigation capability comprising: providing a component;applying an environmental barrier coating to the component, theenvironmental barrier coating comprising an integrated CMAS mitigationlayer including: at least one outer layer material selected from thegroup consisting of BSAS, rare earth monosilicates, rare earthdisilicates, and combinations thereof; and a CMAS mitigation compositionselected from the group consisting of zinc aluminate spinel, alkalineearth zirconates, alkaline earth hafnates, hafnium silicate, zirconiumsilicate, rare earth gallates, rare earth phosphates, tantalum oxide,beryl, alkaline earth aluminates, rare earth aluminates, andcombinations thereof.

Embodiments herein also generally relate to methods making componentshaving CMAS mitigation capability comprising: providing a component;applying an environmental barrier coating to the component, the barriercoating comprising: a bond coat layer comprising silicon; an optionalsilica layer; at least one transition layer comprising a compositionselected from the group consisting of mullite, barium strontiumaluminosilicate (BSAS), and combinations thereof; an optional outerlayer comprising an outer layer material selected from the groupconsisting of BSAS, rare earth monosilicates, rare earth disilicates,and combinations thereof; and a CMAS mitigation composition wherein theCMAS mitigation composition is selected from the group consisting ofzinc aluminate spinel, alkaline earth zirconates, alkaline earthhafnates, rare earth gallates, beryl, and combinations thereof when theCMAS mitigation composition is included as a separate CMAS mitigationlayer, and wherein the CMAS mitigation composition is selected from thegroup consisting of zinc aluminate spinel, alkaline earth zirconates,alkaline earth hafnates, hafnium silicate, zirconium silicate, rareearth gallates, rare earth phosphates, tantalum oxide, beryl, alkalineearth aluminates, rare earth aluminates, and combinations thereof whenthe CMAS mitigation composition is included as an integrated CMASmitigation layer further comprising at least one outer layer material.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating in accordance with the description herein;

FIG. 2 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating having a separate CMAS mitigation layer inaccordance with the description herein; and

FIG. 3 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating having an integrated CMAS mitigation layerin accordance with the description herein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to methods for makingenvironmental barrier coatings and ceramic components having CMASmitigation capability.

The CMAS mitigation compositions described herein may be suitable foruse in conjunction with EBCs for substrates comprising CMCs, andmonolithic ceramics. As used herein, “CMCs” refers tosilicon-containing, or oxide-oxide, matrix and reinforcing materials.Some examples of CMCs acceptable for use herein can include, but shouldnot be limited to, materials having a matrix and reinforcing fiberscomprising non-oxide silicon-based materials such as silicon carbide,silicon nitride, silicon oxycarbides, silicon oxynitrides, and mixturesthereof. Examples include, but are not limited to, CMCs with siliconcarbide matrix and silicon carbide fiber; silicon nitride matrix andsilicon carbide fiber; and silicon carbide/silicon nitride matrixmixture and silicon carbide fiber. Furthermore, CMCs can have a matrixand reinforcing fibers comprised of oxide ceramics.

Specifically, the oxide-oxide CMCs may be comprised of a matrix andreinforcing fibers comprising oxide-based materials such as aluminumoxide (Al₂O₃), silicon dioxide (SiO₂), aluminosilicates, and mixturesthereof. Aluminosilicates can include crystalline materials such asmullite (3Al₂O₃ 2SiO₂), as well as glassy aluminosilicates.

As used herein, “monolithic ceramics” refers to materials comprisingonly silicon carbide, only silicon nitride, only alumina, only silica,or only mullite. Herein, CMCs and monolithic ceramics are collectivelyreferred to as “ceramics.”

As used herein, the term “barrier coating(s)” refers to environmentalbarrier coatings (EBCs). The barrier coatings herein may be suitable foruse on ceramic substrate components 10 found in high temperatureenvironments, such as those present in gas turbine engines. “Substratecomponent” or simply “component” refers to a component made from“ceramics,” as defined herein.

More specifically, EBC 12 may generally comprise any existingenvironmental barrier coating system that generally comprises a siliconbond coat layer 14, an optional silica layer 15 adjacent to bond coatlayer 14, at least one transition layer 16 adjacent to bond coat layer14 (or silica layer 15 if present), an optional outer layer 18 adjacentto transition layer 16, and an optional abradable layer 22 adjacent totransition layer 16 (or outer layer 18 if present), as shown generallyin FIG. 1. As defined previously herein, “transition layer” 16 refers toany of mullite, BSAS, a rare earth disilicate, and various combinationsthereof, while “outer layer” 18 refers to any of the “outer layermaterials” of BSAS, rare earth monosilicates, rare earth disilicates,(collectively referred to herein as “rare earth silicates”) andcombinations thereof, unless specifically noted otherwise.

Bond coat layer 14, optional silica layer 15, transition layer 16,optional outer layer 18, and optional abradable layer 22 may be madeusing conventional methods known to those skilled in the art and appliedas described herein below.

Unlike existing EBCs, and in addition to the layers describedpreviously, the present embodiments also include CMAS mitigationcompositions to help prevent the EBC from degradation due to reactionwith CMAS in high temperature engine environments. Such CMAS mitigationcompositions may be present as a separate CMAS mitigation layer on topof the existing EBC systems, or as an integrated CMAS mitigation layer,as defined herein below.

As shown in FIG. 2, when CMAS mitigation is included in the EBC as aseparate CMAS mitigation layer 20 on top of existing systems, “separateCMAS mitigation layer” 20 refers to compositions selected from zincaluminate spinel (ZnAl₂O₄), alkaline earth zirconates (AeZrO₃), alkalineearth hafnates (AeHfO₃), rare earth aluminates (Ln₃Al₅O₁₂, Ln₄Al₂O₉),rare earth gallates (Ln₃Ga₅O₁₂, Lna₄Ga₂O₉), beryl, and combinationsthereof.

As used herein, “Ae” represents the alkaline earth elements of magnesium(Mg), calcium (Ca), strontium (Sr), barium (Ba), and mixtures thereof.Additionally, as used herein throughout, “Ln” refers to the rare earthelements of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), andmixtures thereof, while “Lna” refers to the rare earth elements oflanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), andmixtures thereof. In one embodiment, CMAS mitigation layer 20 maycomprise up to about 40% porosity, and in another embodiment less thanabout 10% porosity.

By way of example and not limitation, when including a separate CMASmitigation layer 20, the EBC may comprise one of the followingarchitectures: a silicon bond coat layer 14, an optional silica layer15, a mullite-BSAS transition layer 16, an outer layer 18, a separateCMAS mitigation layer 20, and optionally, an abradable layer 22; asilicon bond coat layer 14, an optional silica layer 15, a rare earthdisilicate transition layer 16, an outer layer 18, a separate CMASmitigation layer 20, and optionally, an abradable layer 22; a siliconbond coat layer 14, an optional silica layer 15, a mullite transitionlayer 16, an outer layer 18, a separate CMAS mitigation layer 20, andoptionally, an abradable layer 22; a silicon bond coat layer 14, anoptional silica layer 15, a rare earth disilicate transition layer 16, aseparate CMAS mitigation layer 20, and optionally, an abradable layer22; a silicon bond coat layer 14, an optional silica layer 15, a rareearth disilicate transition layer 16, a BSAS transition layer 16, a rareearth disilicate transition layer 16, a separate CMAS mitigation layer20, and optionally, an abradable layer 22; a silicon bond coat layer 14,an optional silica layer 15, a rare earth disilicate transition layer16, a BSAS transition layer 16, a rare earth disilicate transition layer16, an outer layer 18, a separate CMAS mitigation layer 20, andoptionally, an abradable layer 22; a silicon bond coat layer 14, anoptional silica layer 15, a rare earth disilicate transition layer 16, aBSAS transition layer 16, an outer layer 18, a separate CMAS mitigationlayer 20, and optionally, an abradable layer 22.

In the previous examples, optional abradable layer 22 may comprise thesame material present in separate CMAS mitigation layer 20, a rare earthdisilicate (Ln₂Si₂O₇), or BSAS. The abradable may be a highly porouslayer comprising up to about 50% porosity, or it may consist ofpatterned ridges that are dense (less than about 10% porosity) or porous(up to about 50% porosity). Abradable layer 22 can abrade upon impactfrom an adjacent, rotating engine component. The energy absorbed intothe abradable coating can help prevent damage from incurring to theadjacent, rotating engine component. For example, in one embodiment, theEBC plus abradable layer could be present on a CMC shroud. Adjacentrotating blades having a tight clearance with the shroud could result inan impact event. The presence of abradable layer 22 can help preventdamage to the rotating blades.

As shown in FIG. 3, and as previously described, CMAS mitigation mayalternately be included as an integrated CMAS mitigation layer 120. Inthis instance, “integrated CMAS mitigation layer” 120 refers to a layercomprising CMAS mitigation compositions in combination with any of theouter layer materials. More particularly, the CMAS mitigationcomposition can be included as either discrete dispersed refractoryparticles in the outer layer materials, or as a grain boundary phase inthe outer layer materials. As previously defined, the “outer layermaterials” may comprise any of BSAS, rare earth silicates, orcombinations thereof.

As used herein, “integrated CMAS mitigation layer” 120 may include anyof the outer layer materials with the addition of a CMAS mitigationcomposition selected from zinc aluminate spinel (ZnAl₂O₄), alkalineearth zirconates (AeZrO₃), alkaline earth hafnates (AeHfO₃), hafniumsilicate, zirconium silicate, rare earth aluminates (Ln₃Al₅O₁₂,Ln₄Al₂O₉), rare earth gallates (Ln₃Ga₅O₁₂, Lna₄Ga₂O₉), rare earthphosphates (LnPO₄), tantalum oxide, beryl, alkaline earth aluminates(AeAl₁₂O₁₉, AeAl₄O₉), rare earth aluminates (Ln₃Al₅O₁₂ and Ln₄Al₂O₉),and combinations thereof.

By way of example and not limitation, EBCs having an integrated CMASmitigation layer 120 may comprise one of the following architectures: asilicon bond coat layer 14, an optional silica layer 15, a mullite-BSAStransition layer 16, and an integrated CMAS mitigation layer 120; asilicon bond coat layer 14, an optional silica layer 15, a mullitetransition layer 16, and an integrated CMAS mitigation layer 120; asilicon bond coat layer 14, an optional silica layer 15, a rare earthdisilicate transition layer 16, and an integrated CMAS mitigation layer120; a silicon bond coat layer 14, an optional silica layer 15, amullite-BSAS transition layer 16, a BSAS outer layer 18, and anintegrated CMAS mitigation layer 120; a silicon bond coat layer 14, anoptional silica layer 15, a mullite transition layer 16, a BSAS outerlayer 18, and an integrated CMAS mitigation layer 120; a silicon bondcoat layer 14, an optional silica layer 15, a rare earth disilicatetransition layer 16, a BSAS outer layer 18, and an integrated CMASmitigation layer 120.

Regardless of the particular architecture of the EBC with CMASmitigation, the component can be coated using conventional methods knownto those skilled in the art to produce all desired layers andselectively place the CMAS mitigation composition(s) as either aseparate layer, a grain boundary phase, or discrete, dispersedrefractory particles. Such conventional methods can generally include,but should not be limited to, plasma spraying, high velocity plasmaspraying, low pressure plasma spraying, solution plasma spraying,suspension plasma spraying, chemical vapor deposition (CVD), electronbeam physical vapor deposition (EBPVD), sol-gel, sputtering, slurryprocesses such as dipping, spraying, tape-casting, rolling, andpainting, and combinations of these methods. Once coated, the substratecomponent may be dried and sintered using either conventional methods,or unconventional methods such as microwave sintering, laser sinteringor infrared sintering. Unless an abradable layer is present, the CMASmitigation layer, whether separate or integrated, can be the outermostlayer of the EBC.

More specifically, dispersion of the refractory particles into the outerlayer can occur by various means depending on the process chosen todeposit the barrier coating. For a plasma spray process, particles ofany of the outer layer materials can be mixed with the CMAS mitigationrefractory particles before coating deposition. Mixing may consist ofcombining the outer layer material and the refractory particles withouta liquid, or by mixing a slurry of the outer layer material andrefractory particles. The dry particles or slurries can then bemechanically agitated using a roller mill, planetary mill, blender,paddle mixer, ultrasonic horn, or any other method known to thoseskilled in the art. For the slurry process, the refractory particlesdispersed in the slurry will become dispersed particles in the coatingafter drying and sintering of a slurry-deposited layer.

In order to maintain discrete, refractory particles in themicrostructure, the average particle size of the CMAS mitigationrefractory particles in the slurry can be greater than about 20 nm, andin one embodiment from about 200 nm to about 10 micrometers in size. Therefractory particles can comprise from about 1% to about 60% by volumeof the layer, with the remainder being outer layer material, or outerlayer material and porosity.

The CMAS mitigation grain boundary phase can be produced in a variety ofways, including particle coating and slurry methods. In one example, theCMAS grain boundary phase can be achieved by coating particles of anouter layer material with the desired CMAS mitigation composition(s)before the outer layer material is deposited on the ceramic substrateusing a conventional method known to those skilled in the art. Coatingthe BSAS or rare earth silicate particles can be accomplished bychemical vapor deposition on particles in a fluidized bed reactor or bya solution (sol-gel) type process where precursors of the CMASmitigation composition are deposited onto the outer layer materialparticles from a liquid phase, followed by heat treatment of the BSAS orrare earth silicate particles to form the desired CMAS mitigationcomposition on the surface of the BSAS or rare earth silicate particles.Once the BSAS or rare earth silicate particles with the CMAS mitigationcomposition are obtained, the substrate component can be coated, dried,and sintered using any of the previously described methods known tothose skilled in the art. Ultimately, the surface layer of the CMASmitigation composition on the BSAS or rare earth silicate particlesbecomes the grain boundary phase in the coating. In these instances, toform the grain boundary phase, the refractory particles can have anaverage size of less than about 100 nm. If the grain boundary particlesare larger than about 100 nm, they will be dispersed in the outer layeras described previously rather than forming a grain boundary phase. Ifthe grain boundary particles are larger than about 100 nm, they will bedispersed in the outer layer as described previously rather than servingas a grain boundary phase.

In one embodiment, the sol-gel solution, may be an aqueous solutioncomprised of soluble salts, while in another embodiment, the sol-gelsolution may be an organic solvent solution containing an organic salt.As used herein, “soluble salts” may include, but are not limited to,alkaline earth nitrates, alkaline earth acetates, alkaline earthchlorides, rare earth nitrates, rare earth acetates, rare earthchlorides, aluminum nitrate, aluminum acetate, aluminum chloride,ammonium phosphate, phosphoric acid, polyvinyl phosphonic acid, galliumnitrate, gallium acetate, gallium chloride, zinc nitrate, zinc acetate,zinc chloride, zirconyl chloride, zirconyl nitrate, ammonium tantalumoxalate, ammonium niobium oxalate, beryllium nitrate, beryllium acetate,beryllium chloride, hafnium chloride, hafnium oxychloride hydrate, andcombinations thereof.

As used herein, “organic solvents” may include methanol, ethanol,propanol, butanol, pentanol, hexanol, heptanol, octanol, acetone, methylisobutyl ketone (MIBK), methyl ethyl ketone (MEK), toluene, heptane,xylene, or combinations thereof. As used herein, “organic salts” caninclude aluminum butoxide, aluminum di-s-butoxide ethylacetoacetate,aluminum diisopropoxide ethylacetoacetate, aluminum ethoxide, aluminumethoxyethoxyethoxide, aluminum 3,5-heptanedionate, aluminumisopropoxide, aluminum 9-octadecenylacetoacetate diisopropoxide,aluminum 2,4-pentanedionate, aluminum pentanedionatebis(ethylacetoacetate), aluminum 2,2,6,6-tetramethyl3,5-heptanedionate,and aluminum phenoxide, gallium 8-hydroxyquinolinate, gallium2,4-pentanedionate, gallium ethoxide, gallium isopropoxide, and gallium2,2,6,6-tetramethylheptanedionate, calcium isopropoxide, calciummethoxyethoxide, calcium methoxide, calcium ethoxide, strontiumisopropoxide, strontium methoxypropoxide, strontium 2,4-pentanedionate,strontium 2,2,6,6-tetramethyl-3,5-heptanedionate, magnesium ethoxide,magnesium methoxide, magnesium methoxyethoxide, magnesium2,4-pentanedionate, magnesium n-propoxide, barium isopropoxide, bariummethoxypropoxide, barium 2,4-pentanedionate, barium2,2,6,6-tetramethyl-3,5-heptanedionate, rare earth methoxyethoxide, rareearth isopropoxide, rare earth 2,4-pentanedionate, zincN,N-dimethylaminoethoxide, zinc 8-hydroxyquinolinate, zincmethoxyethoxide, zinc 2,4-pentaedianote, zinc2,2,6,6-tetramethyl-3,5-heptanedianate, zirconium butoxide, zirconiumdibutoxide, zirconium diisopropoxide, zirconium dimethacrylatedibutoxide, zirconium ethoxide, zirconium 2-ethylhexoxide, zirconium3,5-heptanedionate, zirconium isopropoxide, zirconiummethacryloxyethylacetoacetate tri-n-butoxide, zirconium2-methyl-2-butoxie, zirconium 2-methoxymethyl-2-propoxide, zirconium2,4-pentanedionate, zirconium n-propoxide, zirconium2,2,6,6-tetramethyl-3,5-heptanedionate, hafnium n-butoxide, hafniumt-butoxide, hafnium di-n-butoxide, hafnium ethoxide, hafnium2-ethylhexoxide, hafnium 2-methoxymethyl-2-propoxide, hafnium2,4-pentanedionate, hafnium tetramethylheptanedionate, niobium Vn-butoxide, niobium V ethoxide, tantalum V n-butoxide, tantalum Vethoxide, tantalum V isopropoxide, tantalum V methoxide, tantalumtetraethoxide dimethylaminoethoxide, tantalum V tetraethoxidepentanedionate, polyvinyl phosphonic acid, polyvinyl phosphoric acid,and combinations thereof.

Regardless of whether the CMAS mitigation composition is present as aseparate mitigation layer on top of the existing EBC systems, or as anintegrated mitigation layer (e.g. discrete dispersed refractoryparticles, or a grain boundary phase), the benefits are the same.Namely, CMAS mitigation compositions can help prevent the EBC fromdegradation due to reaction with CMAS in high temperature engineenvironments. More particularly, CMAS mitigation compositions can helpprevent or slow the reaction of CMAS with the barrier coating that canform secondary phases that rapidly volatilize in steam. Additionally,CMAS mitigation compositions can help prevent or slow the penetration ofCMAS through the barrier coating along the grain boundaries into anonoxide, silicon-based substrate. Reaction of CMAS with substrates suchas silicon nitrate and silicon carbide evolve nitrogen-containing andcarbonaceous gases, respectively. Pressure from this gas evolution canresult in blister formation within the EBC coating. These blisters caneasily rupture and destroy the hermetic seal against water vaporprovided by the EBC in the first instance.

The presence of CMAS mitigation compositions can help prevent or slowthe attack of molten silicates on the EBC, thereby allowing the EBC toperform its function of sealing the CMC from corrosive attack in hightemperature steam. Moreover, CMAS mitigation compositions can helpprevent recession of the CMC, and also any layers of the EBC that may besusceptible to steam recession if CMAS reacts with it, to formsteam-volatile secondary phases. Dimensional changes of ceramiccomponents due to steam recession can limit the life and/orfunctionality of the component in turbine engine applications. Thus,CMAS mitigation is important to allow the barrier coating to perform itsfunctions; thereby allowing the CMC component to function properly andfor its intended time span.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A method of making a component having calciummagnesium aluminosilicate (CMAS) mitigation capability, the methodcomprising: providing a component comprising a ceramic matrix compositeor a monolithic ceramic; applying an environmental barrier coating tothe component, the environmental barrier coating comprising: a bond coatlayer comprising silicon overlying the component; an optional silicalayer overlying the bond coat layer; at least one transition layeroverlying the bond coat layer or the optional silica layer comprising acomposition selected from the group consisting of mullite, bariumstrontium aluminosilicate (BSAS), rare earth disilicates, andcombinations thereof; an optional outer layer comprising an outer layermaterial selected from the group consisting of BSAS, rare earthmonosilicates, rare earth disilicates, and combinations thereof; and aseparate CMAS mitigation layer overlying the at least one transitionlayer or the optional outer layer, wherein the separate CMAS mitigationlayer consists of Lna₄Ga₂O₉ and optionally one or more compositionsselected from the group consisting of zinc aluminum spinel, alkalineearth zirconates, alkaline earth hafnates, rare earth aluminates, rareearth gallates, beryl, and combinations thereof, wherein the separateCMAS mitigation layer contains Lna₄Ga₂O₉ in an amount sufficient toprevent degradation of the environmental barrier coating due to reactionwith CMAS.
 2. The method of claim 1, further comprising: applying anabradable layer to the separate CMAS mitigation layer.
 3. The method ofclaim 2, wherein the abradable layer comprises Lna₄Ga₂O₉, a rare earthdisilicate, or BSAS.
 4. The method of claim 1, wherein the environmentalbarrier coating is applied by a method selected from the groupconsisting of plasma spraying, low pressure plasma spraying, solutionplasma spraying, suspension plasma spraying, chemical vapor deposition,electron beam physical vapor deposition, sol-gel, sputtering, slurrydipping, slurry spraying, slurry painting, slurry rolling, tape-casting,and combinations thereof.
 5. The method of claim 1, wherein thecomponent is a turbine engine component selected from the groupconsisting of combustor components, turbine blades, shrouds, nozzles,heat shields, and vanes.
 6. A method of making a component having CMASmitigation capability, the method comprising: providing a component,wherein the component comprises a ceramic matrix composite or amonolithic ceramic; applying an environmental barrier coating to thecomponent, the environmental barrier coating comprising a bond coatlayer comprising silicon overlying the component; an optional silicalayer overlying the bond coat layer; at least one transition layeroverlying the bond coat layer or the optional silica layer comprising acomposition selected from the group consisting of mullite, bariumstrontium aluminosilicate (BSAS), rare earth disilicates, andcombinations thereof; an optional outer layer overlying the at least onetransition layer, the optional outer layer comprising at least one outerlayer material selected from the group consisting of BSAS, rare earthmonosilicates, rare earth disilicates, and combinations thereof; and anintegrated CMAS mitigation layer overlying the at least one transitionlayer or the optional outer layer including: at least one outer layermaterial; and a CMAS mitigation composition comprising Lna₄Ga₂O₉,wherein the integrated CMAS mitigation layer contains Lna₄Ga₇O₉ in anamount sufficient to prevent degradation of the environmental barriercoating due to reaction with CMAS.
 7. The method of claim 6, furthercomprising: applying an abradable layer to the integrated CMASmitigation layer.
 8. The method of claim 7, wherein the abradable layercomprises Lna₄Ga₂O₉, a rare earth disilicate, or BSAS.
 9. The method ofclaim 6, wherein the environmental barrier coating is applied by amethod selected from the group consisting of plasma spraying, lowpressure plasma spraying, solution plasma spraying, suspension plasmaspraying, chemical vapor deposition, electron beam physical vapordeposition, sol-gel, sputtering, slurry dipping, slurry spraying, slurrypainting, slurry rolling, tape-casting, and combinations thereof. 10.The method of claim 6, wherein the integrated CMAS mitigation layercomprises the CMAS mitigation composition as a grain boundary phase onthe outer layer material or as dispersed refractory particles in theouter layer material.
 11. The method of claim 6, wherein the componentis a turbine engine component selected from the group consisting ofcombustor components, turbine blades, shrouds, nozzles, heat shields,and vanes.
 12. A method of making a component having CMAS mitigationcapability comprising: providing a component; applying an environmentalbarrier coating to the component, the barrier coating comprising: a bondcoat layer comprising silicon; an optional silica layer; at least onetransition layer comprising a composition selected from the groupconsisting of mullite, barium strontium aluminosilicate (BSAS), andcombinations thereof; an optional outer layer comprising an outer layermaterial selected from the group consisting of BSAS, rare earthmonosilicates, rare earth disilicates, and combinations thereof; and aseparate CMAS mitigation layer consisting of Lna₄Ga₂O₉ and optionallyone or more compositions selected from the group consisting of zincaluminum spinel, alkaline earth zirconates, alkaline earth hafnates,rare earth aluminates, rare earth gallates, beryl, and combinationsthereof or an integrated CMAS mitigation layer comprising Lna₄Ga₂O₉ andat least one outer layer material, wherein the separate CMAS mitigationlayer or the integrated CMAS mitigation layer contains Lna₄Ga₂O₉ in anamount sufficient to prevent degradation of the environmental barriercoating due to reaction with CMAS.
 13. The method of claim 12 furthercomprising: applying an abradable layer to the separate CMAS mitigationlayer or the integrated CMAS mitigation layer.
 14. The method of claim13, wherein the abradable layer comprises Lna₄Ga₂O₉, a rare earthdisilicate, or BSAS.
 15. The method of claim 12, wherein theenvironmental barrier coating is applied by a method selected from thegroup consisting of plasma spraying, low pressure plasma spraying,solution plasma spraying, suspension plasma spraying, chemical vapordeposition, electron beam physical vapor deposition, sol-gel,sputtering, slurry dipping, slurry spraying, slurry painting, slurryrolling, tapecasting, and combinations thereof.
 16. The method of claim12, wherein the integrated CMAS mitigation layer comprises the CMASmitigation composition as a grain boundary phase on the outer layermaterial or as dispersed refractory particles in the outer layermaterial.
 17. The method of claim 12, wherein the component comprises aceramic matrix composite or a monolithic ceramic.
 18. The method ofclaim 12, wherein the component is a turbine engine component selectedfrom the group consisting of combustor components, turbine blades,shrouds, nozzles, heat shields, and vanes.